1- Aminopyridinium cation

Hydroxylamine Osulfonic acid converts pyridine into the 1-aminopyridinium cation. Pyridazines undergo A-amination readily. 1,2,3-Triazine A-imines (80) were obtained from the reaction with O-mesitylenesulfonylhydroxylamine (MSH), followed by a neutralization with potassium carbonate. 

Pyridines react with potassium hydroxylamine O-sulphonate to give salts of 1-aminopyridinium cations ft ... 

A-Aminopyridinium cations can be acylated or sulfonylated (with acid halides) and nitrated (H2S04 - HN03) to give the corresponding -(substituted amino)pyridines (972), often isolated as the imides (973 R = COR, S02R or N02). 

Aldehydes readily yield imines, cf (976), and aryldiazonium salts form aryl azides and pyridine, presumably via ArN = N- N-Py+. Nitrous acid and A-aminopyridinium cations yield pyridine and... 

The amino group in 1018 can be condensed with 1,4-dicarbonyl compounds for example, with hexane-2,S-dione, the Ar-(pyrro 1 -1 -yI)pyridini 11 m 1019 is formed, or with aldehydes, imines 1020 are formed. Reaction with aryldiazonium salts forms aryl azides and pyridine, presumably via ArN=NNPy+. Nitrous acid and At-aminopyridinium cations yield pyridine and N20. 

A bicyclic system, 5-azaindole, may be mentioned here because it owes its relatively high basic strength (p/sTg = 8 3) to a resonance stabilization of the cation [71] which is analogous to that of 4-aminopyridinium ion (Adler and Albert, 1960). Nmr spectra of the cations of 5-azaindole and its 1-phenyl derivatives have recently been reported (Dvoryantseva et al., 1973). 

Abramovitch and Takeuchi had explored the use of N-aminopyridinium ions as precursors to nitrenium ions in both photolysis and thermolysis reactions, but had not attempted to use these precursors under flash photolysis conditions. Moran and Falvey demonstrated that the LFP of 138 in CFI3CN (Scheme 57) generated a short-lived intermediate (1.5 jus) that had characteristics similar to the ions l31b-f. The transient was identified as the singlet ion 116. " Evidence from product analysis and spectrophotometric detection of the cation radical Ph2NH- as a long-lived intermediate was initially thought to indicate that there were parallel path-... 

Because of the similar energy of the two possible resonances that can ensure hydration of the pteridine cation (namely, 4-aminopyridinium stabilization of 7,8-hydration and amidinium stabilization of 3,4-hydration3 ), a study of the effects of substituents on this delicately balanced system was undertaken. An initial survey showed that the majority of pteridine cations were subject neither to dihydration nor to shifting hydration. Only four examples were found in which 5,6,7,8-dihydration was preceded by 3,4-hydration, namely 2-methyl-, 6-methyl-, 7-methyl-, and 2-amino-4-methylpteridine, whereas the related cations of 6,7-dimethyl-, 2-amino-, 2-amino-6-methyl-, and 2-amino-7-methylpteridine remained stably hydrated in the 3,4-position.8,40 Further examples, including a reversed direction of shift, came to light from a... 

The reaction of N-aminoazonium salts with aliphatic and aromatic aldehydes in the absence of base gives the Schiff base type compounds 28.151,153-156 Some aliphatic ketones also react with N-aminoazonium salts.40,155 Hexane-2,5-dione reacts with N-aminopyridinium perchlorate to give l-(l -pyrrolyl)pyridinium cation 29 together with some of the bisper-chlorate 30.157 Certain esters such as diethyl malonate and ethyl cyano-acetate react with N-aminopyridinium salts in the presence of base to give the corresponding N-acylimines 31.48 However, the reaction of ethyl aceto-acetate and acetylacetone with N-aminoazonium salts in the presence of base gives 1,3-dipolar cycloaddition products (Section IV,C,1).36,154,158 The reaction of ethyl acetoacetate with 1-alkyl-l,2,4-triazole 4-imine affords zwitterionic triazolo[4,3-/>]pyridazines 32.139,159... 

Acridine 2.16) is a weak base. With its pA a of5.3, it is only 1% ionized at pH 7.3. It turned out that the 3-amino- and 9-amino-acridines had a resonance in their cation that was lacking in the neutral species [e.g. 2.17) <—> 2.IS) and 2.19) <—> 2.20), respectively and this made them very strong bases. The other three aminoacridines could not, for reasons of valence, acquire this resonance stabilization and, hence, were very little stronger than acridine itself (see Table 2.2). We called this the 4-aminopyridinium type of base-strengthening resonance, because we had demonstrated it first in 4-aminopyridine. 

The salt bis(4-aminopyridinium) tetrachloropaUadate(II) is formed by a discrete PdCIj anion and two Np, protonated C5H7N2 cations withont appreciable Pd—N interactions (Figure 4.1). The geometry of Pd(II) ion is square planar (Pd-Cl(l),... 

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